Of memory integrated circuits, switching elements having a nonvolatile function capable of being kept on or off even when their power supply is switched off include an antifuse element as a first conventional example and an EEPROM (Electrical Erasable Programmable Read Only Memory) as a second conventional example.
Switching elements for performing a nonvolatile function based on an electrochemical reaction include a timer (or an electrochemical time switching device) as a third conventional example and a PCRAM (Programmable Conductor Random Access Memory) as a fourth conventional example.
The antifuse element as the first conventional example is a switching element having two states, i.e., electrically on and off states, and can irreversibly transit from the off state to the on state according to an electrical or physical process. The antifuse element as the first conventional example is disclosed in U.S. Pat. No. 5,070,384 and U.S. Pat. No. 5,387,812. The antifuse element is usually formed between two interconnects. When a high voltage is selectively applied between the interconnects, the antifuse element is programmed (transiting from the off state to the on state), electrically interconnecting the interconnects. Even after the voltage is turned off, the antifuse element remains in the on state.
The EEPROM as the second conventional example as disclosed in U.S. Pat. No. 4,203,158 has a floating gate electrode disposed between the control gate electrode and channel layer of a transistor. When the floating gate electrode stores electric charges, i.e., when it is charged, or when the floating gate electrode discharges electric charges, i.e., when it is discharged, the threshold voltage of the transistor changes. The floating gate electrode is charged or discharged by injecting electrons into the floating gate electrode or discharging electrons from the floating gate electrode in the form of a tunnel current flowing through an oxide film. Since the floating gate electrode is surrounded by an insulating film, the electric charges stored therein are not lost after the EEPROM is switched off. Therefore, the EEPROM has a nonvolatile capability.
In recent years, antifuse elements and EEPROMs are used in FPL circuits which are integrated circuits whose hardware configuration can be changed for each application. One example of an FPL circuit is disclosed in Japanese laid-open patent publication No. 8-78532. The disclosed FPL circuit has a plurality of logic circuit blocks, interconnects interconnecting the logic circuit blocks, and antifuse elements for changing the connection of the interconnects. The antifuse elements are used as programming elements. The antifuse elements selected by the user connect interconnects. Therefore, the FPL circuit provides a different hardware configuration for a selection of antifuse elements to connect interconnects. FPL circuits offer many advantages in that they are more versatile than ASICs (Application Specific Integrated Circuits) and can be manufactured inexpensively in a short turnaround time, and are finding a rapidly growing market.
The timer as the third conventional example has a closed loop made up of a DC power supply, a load, and first and second inner electrodes. Part of the first and second inner electrodes is immersed in an electrolytic solution and electroplated, and one of the first and second inner electrodes is cut off, setting a time for the timer. The timer as the third conventional example is disclosed in Japanese laid-open utility model publication No. 2-91133.
The electronic element as the fourth conventional example, as disclosed in U.S. Pat. No. 6,348,365, is a PCRAM utilizing silver germanide/selenide which is a silver ion conductive ion conductive material (the term “ion conductive material” has the same meaning as “ion conductor” used in the present specification) as a material for conducting ions.
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing a structure of the PCRAM disclosed in U.S. Pat. No. 6,348,365. As shown in FIG. 1, insulating material 81, conductive material 82, and dielectric material 83 are successively disposed on semiconductor substrate 87, and dielectric material 83 partly has a recessed structure (grooved structure). Ion conductive material 86 and metal material 84 are disposed in the recessed structure, and electrode 85 is disposed on metal material 84 and dielectric material 83. When a voltage is applied between electrode 85 and conductive material 82, a current path referred to as a dendrite grows on the surface of ion conductive material 86, thus electrically connecting electrode 85 and conductive material 82 to each other. When a reverse voltage is applied, the dendrite disappears, electrically isolating electrode 85 and conductive material 82 from each other.
The antifuse element as the first conventional example is a switching element used mainly in FPL circuits. Since the on-resistance, which is the resistance of the antifuse element when it is in the on state, is small (about 50Ω), the antifuse element has a small signal delay time. However, the antifuse element is problematic in that it is not reprogrammable. Consequently, when the FPL circuit is programmed, it fails to meet demands for debugging the program and changing programs while it is in operation.
While the EEPROM as the second conventional example is reprogrammable, the level of integration thereof is low at present and the on-resistance thereof is of a large value of several kΩ because it is limited by the resistance of the MOS (Metal Oxide Semiconductor) transistor. Though the EEPROM is widely used as nonvolatile memory, the level of integration thereof is limited by the thickness of the insulating film, making it difficult to further integrate the EEPROM. In addition, when the EEPROM is used in an FPL circuit, it tends to cause a signal delay due to the large on-resistance.
The timer as the third conventional example is a device for measuring time until the electrode is dissolved by an electroplating process which is based on an electrochemical reaction. The timer cannot operate as a switching element for switching between on and off states.
The electronic element as the fourth conventional example is basically a two-terminal switch utilizing an electrochemical reaction. The transition of the two-terminal switch between on and off states is controlled by a voltage that is applied between the two terminals of the switch. When the transition occurs between the on and off states, a current flows through the switch, and the switch consumes a large amount of electric power. The switch requires thick interconnects that can withstand the current that is needed to cause the transition between the on and off states, and also requires a transistor having large drive power. Even though the switch itself can be integrated, it is difficult to integrate the interconnects and peripheral circuits.